Toner density sensor and image forming apparatus

ABSTRACT

A toner density sensor has a light emitting unit that emits light to detect toner density, a light receiving unit that receives the light emitted from the light emitting unit and reflected from a detection target, and a board on which the light emitting unit and the light receiving unit are surface-mounted. A penetration space that penetrates the board in a thickness direction is formed in at least one of portions in which the light emitting unit and the light receiving unit are attached to the board.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a toner density sensor that is used inan image forming apparatus such as a copying machine, a printer, and afacsimile Machine, particularly to a toner density sensor that canimprove detection accuracy.

2. Related Art

The toner density sensor is a main component that is used to acquireoptimum image quality in the image forming apparatus. The toner densitysensor includes a light emitting unit that emits light, a lightreceiving unit that receives the light, which is emitted from the lightemitting unit and reflected from a detection target, and an amplifyingunit that amplifies a detection voltage of the light receiving unit. Inthe case of an intermediate transfer type image forming apparatus inwhich a toner image primarily transferred to an intermediate transferbelt is secondarily transferred to a paper sheet, in the toner densitysensor, when the light emitting unit emits the light to the intermediatetransfer belt, the light receiving unit detects the light reflected fromthe toner image on the intermediate transfer belt. Toner densityadhering to the intermediate transfer belt is detected based on aphotocurrent (detection voltage) generated in the light receiving unit,and a necessary correction is optically or electrically performed basedon a detection result of the toner density.

However, the light emitting unit and the light receiving unit of thetoner density sensor are surface-mounted on a printed board, and thelight is emitted from the light emitting unit in directions except adesired direction.

Therefore, noise light is generated. The noise light is also calledstray light, which causes degradation of the detection accuracy. Notonly the light emitted from the light emitting unit surface-mounted onthe board travels toward the desired detection target, but also thelight invades in the board. In the board made of paper and a phenolresin or glass and an epoxy resin or the like, the light travels whilebeing reflected, and the light reaches a surrounding area of the lightreceiving unit. As a result, a noise is generated in the detectionvoltage of the light receiving unit, and the detection is hardlyperformed with high accuracy.

For example, Japanese Unexamined Patent Publication No. 2009-58520discloses a toner density sensor.

In the configuration of Japanese Unexamined Patent Publication No.2009-58520, an elongate slit-shaped through-hole is provided between thelight emitting unit and the light receiving unit, which aresurface-mounted on the board of the sensor.

The light that invades and propagates in the board from the lightemitting unit is eliminated such that the light passes through thethrough-hole, thereby reducing the noise light reaching the lightreceiving unit.

According to the configuration of Japanese Unexamined Patent PublicationNo. 2009-58520, the noise light can be reduced. However, it is necessaryto ensure an area in which the through-hole is made (see FIG. 4 ofJapanese Unexamined Patent Publication No. 2009-58520). Therefore, asillustrated in a toner density sensor 101 in FIG. 15, the configurationof Japanese Unexamined Patent Publication No. 2009-58520 cannot beadopted in a case where a light emitting unit 102 and light receivingunits 103 and 104 are brought close to each other in order to achieveminiaturization. In FIG. 15, the numeral 105 designates a printed board,the numeral 106 designates a case, and the numeral 107 designates alens.

SUMMARY

One or more embodiments of the present invention prevents thedegradation of the detection accuracy, which is caused by the noiselight, even in a close distance between the light emitting unit and thelight receiving unit.

In accordance with one or more embodiments of the present invention, atoner density sensor includes: a light emitting unit that emits light inorder to detect toner density; and a light receiving unit that receivesthe light, which is emitted from the light emitting unit and reflectedfrom a detection target, wherein the light emitting unit and the lightreceiving unit are surface-mounted on a board, and a penetration spacethat penetrates the board in a thickness direction is formed in at leastone of portions in which the light emitting unit and the light receivingunit are attached to the board.

In the configuration of the toner density sensor, when the penetrationspace is formed in the portion in which the light emitting unit isattached to the board, the light that is emitted from the light emittingunit to possibly become the noise light radiates to the outside throughthe penetration space, and the light propagating in the board isreduced. When the penetration space is formed in the portion in whichthe light receiving unit is attached to the board, the noise light thatpropagates in the board to reach the light receiving unit is diffused bythe inside surface of the penetration space, and the noise lightreaching the light receiving unit is reduced.

According to one or more embodiments of the invention, the penetrationspace prevents the noise light from being generated or reaching thelight receiving element, so that the detection accuracy of the lightreceiving unit can be, improved. The penetration space is formed in theportion in which the surface-mounted light emitting unit or lightreceiving unit is attached to the board, so that the necessity to ensurethe additional flat surface is eliminated, and the small area caneffectively be utilized. Therefore, one or more embodiments of theinvention can suitably be applied to the small-size toner densitysensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a toner density sensor;

FIG. 2A is a front view schematically illustrating the toner densitysensor, and FIG. 2B is a sectional view schematically illustrating thetoner density sensor;

FIG. 3 is a schematic configuration diagram of an image formingapparatus;

FIG. 4A is a plan view illustrating a structure of the toner densitysensor, and FIGS. 4B and 4C are sectional views illustrating thestructure of the toner density sensor;

FIG. 5 is a plan view illustrating another example of the printed board;

FIG. 6 is a sectional view illustrating another example of the tonerdensity sensor;

FIG. 7 is a sectional view illustrating still another example of thetoner density sensor;

FIG. 8 is a sectional view illustrating still another example of thetoner density sensor;

FIG. 9A is a plan view illustrating still another example of thestructure of a toner density sensor, and FIGS. 9B and 9C are sectionalviews illustrating still another example of the structure of the tonerdensity sensor;

FIG. 10 is a sectional view illustrating still another example of thetoner density sensor;

FIG. 11A is a plan view illustrating still another example of thestructure of a toner density sensor, and FIGS. 11B and 11C are sectionalviews illustrating still another example of the structure of the tonerdensity sensor;

FIG. 12A is a plan view illustrating still another example of thestructure of a toner density sensor, and FIGS. 12B and 12C are sectionalviews illustrating still another example of the structure of the tonerdensity sensor;

FIG. 13 is a sectional view illustrating still another example of thetoner density sensor;

FIG. 14 is a sectional view illustrating still another example of thetoner density sensor; and

FIG. 15 is a transverse sectional view illustrating a case portion of aconventional toner density sensor.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. In embodiments of the invention, numerousspecific details are set forth in order to provide a more thoroughunderstanding of the invention. However, it will be apparent to one ofordinary skill in the art that the invention may be practiced withoutthese specific details. In other instances, well-known features have notbeen described in detail to avoid obscuring the invention.

FIG. 1 is a perspective view of a toner density sensor 11, and FIG. 2 isan explanatory view illustrating a schematic structure of the tonerdensity sensor 11.

The toner density sensor 11 is mounted on an image forming apparatus 51illustrated in FIG. 3. For example, the image forming apparatus 51 is acolor laser printer. The schematic structure of the image formingapparatus 51 will be described below.

The image forming apparatus 51 includes an original reading unit 52 thatis provided in an upper portion thereof, an image forming unit 53, asheet feed unit 54 that is provided in a lower portion, and a sheetdischarge unit 55 that is provided in the upper portion. In the imageforming apparatus 51, the image forming unit 53 forms an image based onoriginal data read with the original reading unit 52, the image istransferred to a paper sheet 54 a supplied from the sheet feed unit 54,and the paper sheet 54 a is discharged from the sheet discharge unit 55.A transfer belt 56 is tensioned in the image forming unit 53. Toneradheres to a photosensitive drum 58 that is exposed to light from alight writing device 57, and the toner is primarily transferred to thetransfer belt 56 to form the image. When the paper sheet 54 a issupplied, the image is secondarily transferred from the transfer belt 56to the paper sheet 54 a. Then the paper sheet 54 a is conveyed to afixing unit 59, and the toner is fixed to the paper sheet 54 a by heatand a pressure.

An image forming unit 63 includes a charging roller 60, a developmentsleeve 61, a toner case 62, and the photosensitive drum 58. In the imageforming unit 63, a yellow image forming unit 63Y, a magenta imageforming unit 63M, a cyan image forming unit 63C, and a black imageforming unit 63B are provided.

The toner density sensor 11 is provided opposite the transfer belt 56 inthe image forming apparatus 51, and detects the toner density on thetransfer belt 56. The toner density sensor 11 may be provided in theimage forming unit 63. In this case, the toner density sensor 11 detectsthe toner density on the photosensitive drum 58.

The toner density sensor 11 will be described below.

As illustrated in FIG. 2A, the toner density sensor 11 includes a lightemitting element 12 that is the light emitting unit emitting the light,light receiving elements 13 and 14 that are the light receiving unitreceiving the light, which is emitted from the light emitting element 12and reflected from the transfer belt 56 that is the detection target,and an amplifier circuit (not illustrated) that amplifies detectionvoltages of the light receiving elements 13 and 14. For example, a lightemitting diode is used as the light emitting element 12, and aphototransistor or a photodiode is used as the light receiving elements13 and 14.

The light emitting element 12 and the light receiving elements 13 and 14are surface-mounted on a printed board 15 (see FIG. 2B).

A portion in which the light emitting element 12 and the light receivingelements 13 and 14 are mounted is covered with a case 16. As illustratedin FIGS. 1 and 2B, the case 16 includes an upper case 17 and a lowercase 18, and a lens member 19 is retained in a portion on an edge sideof the printed board. The side on which the light emitting element 12and the light receiving elements 13 and 14 are mounted is covered withthe upper case 17, and a surface on the opposite side of the printedboard 15 is covered with the lower case 18.

Specifically, as illustrated by a broken line of FIG. 2A, the lightemitting element 12 and the light receiving elements 13 and 14 aredisposed on the substantially straight line. In the light receivingelements 13 and 14, the first light receiving element 13 located on theleft side in FIG. 2A receives the regularly reflected light in thelight, which is emitted from the light emitting element 12 and reflectedfrom the transfer belt 56, and the first light receiving element 13mainly detects the density of black toner. In the light receivingelements 13 and 14, the second light receiving element 14 located on theright side in FIG. 2A receives the diffusely reflected light in thelight, which is emitted from the light emitting element 12 and reflectedfrom the transfer belt 56, and the second light receiving element 14mainly detects the density of yellow, magenta, and cyan color toner.

As illustrated in FIG. 2B, in order to improve the detection accuracy,the toner density sensor 11 has a configuration in which a penetrationspace 21 that penetrates the printed board 15 in a thickness directionis formed in a portion in which at least one of the light emittingelement 12 and the light receiving elements 13 and 14 is attached to theprinted board 15. The penetration space 21 prevents the generation ofthe noise light invading in the printed board 15, or prevents the noiselight invading in the printed board 15 from reaching the light receivingelements 13 and 14.

The toner density sensor 11 is configured as illustrated in FIG. 4 whenthe penetration space 21 is formed in the portion in which the lightemitting element 12 is attached to the printed board 15. A wiringpattern except a land 15 a that is a soldering copper foil used to mountthe light emitting element 12 and the light receiving elements 13 and 14on the surface of the printed board 15 is not illustrated in FIG. 4. Thesame holds true for the following drawings.

As illustrated in FIG. 4A, the hole-shaped penetration space 21, whichpenetrates the printed board 15 in the thickness direction, is formed inthe portion in which the light emitting element 12 is attached. Thepenetration space 21 has a rectangular shape, when viewed from theabove. The penetration space 21 is formed while including a portioncorresponding to a chip 12 a of the light emitting element 12 (see FIGS.4 b and 4 c).

A shape and a size of the penetration space 21 are properly set, and itis only necessary to form the penetration space 21 in the portioncorresponding to the chip 12 a of the light emitting element 12. In thecase of the small-size penetration space 21, the penetration space 21may be formed around a region corresponding to the chip 12 a.

The shape and the size of the penetration space 21 are properly set inconsideration of the land 15 a.

The penetration space 21 is not formed in the portions in which thefirst light receiving element 13 and the second light receiving element14 are attached.

In the case 16, a through-hole 22, which is the hole portion penetratingthe printed board 15 in the thickness direction, is also made in thelower case 18 with which the lower surface of the printed board 15 iscovered. As illustrated in FIGS. 4B and 4C, the through-hole 22 is madein the region corresponding to the penetration space 21 of the printedboard 15.

In FIGS. 4B and 4C, the through-hole 22 in the lower case 18 is madelarger than the penetration space 21 of the printed board 15.Alternatively, the through-hole 22 may be made equal to or smaller thanthe penetration space 21.

FIG. 4B is a transverse sectional view illustrating the upper case 17 inthe toner density sensor 11, in which the light emitting element 12 andthe light receiving elements 13 and 14 are surface-mounted and the case16 is attached.

In the toner density sensor 11 having the above configuration, the lightemitted from the light emitting element 12 travels toward the directionof the lens member 19 as illustrated in FIG. 4B, and the light alsotravels in the direction of the printed board 15 as illustrated in FIG.4C.

Emitted light L1 travelling in the direction of the lens member 19 istransmitted through the lens member 19, and reflected by the transferbelt 56. Reflected light L2 is received by the light receiving elements13 and 14 through the lens member 19. In FIG. 4C, only the first lightreceiving element 13 is illustrated while the second light receivingelement 14 is not illustrated. However, the same holds true for thesecond light receiving element 14. The same holds true for the followingdrawings.

Based on a detection voltage of the reflected light L2, the tonerdensity is detected as described above.

On the other hand, the light travelling from the light emitting element12 in the direction of the printed board 15 radiates to the outsidethrough the penetration space 21 of the printed board 15 and thethrough-hole 22 of the lower case 18.

Although part of the emitted light invades in the printed board 15,since emitted light L3 travelling in the direction of the printed board15 radiates substantially from the penetration space 21, the amount ofnoise light invading in the printed board 15 can be reduced. Even if thesmall amount of noise light invades in the printed board 15, the lightattenuates in time. As a result, the noise light reaching the lightreceiving elements 13 and 14 is significantly reduced.

Accordingly, the light receiving elements 13 and 14 are hardlyinfluenced by the noise light, and the improvement of the detectionaccuracy can be achieved.

The penetration space 21 is formed in the portion in which the lightemitting element 12 is attached. In the printed board 15, because of thestructure in which the penetration space 21 is formed below the lightemitting element 12, the necessity of the additional flat surface inwhich the penetration space 21 is provided is eliminated, and the smallarea can effectively be utilized. Therefore, thehigh-detection-accuracy, small-size toner density sensor 11 can beobtained.

Since the through-hole 22 is made in the lower case 18, the lightemitted from the light emitting element 12 further radiates to theoutside, and the light that possibly becomes the noise light can bereduced.

The toner density sensor 11 has the high detection accuracy, so that thehigh-quality image can be formed in the image forming apparatus 51 onwhich the toner density sensor 11 is mounted. Additionally, the tonerdensity sensor 11 can be miniaturized, the restricted space of the imageforming apparatus 51 can effectively be utilized to contribute to theprovision of the better image forming apparatus.

FIG. 5 illustrates another example of the penetration space 21. Not onlythe penetration space 21 formed into the hole shape, the wholecircumference of which is surrounded, the penetration space 21 may beformed into a shape in which the penetration space 21 reaches an endsurface of the printed board 15, in other words, a shape in which thepenetration space 21 is formed by cutting the printed board 15 from theend surface.

FIG. 6 illustrates still another example in which the through-hole 22 isnot made in the lower case 18. When the through-hole 22 is not made inthe lower case 18, according to one or more embodiments of the presentinvention, a surface 18 a on the side of the printed board 15 in theregion corresponding to the penetration space 21 has a matte blackcolor. The matte black color can absorb the emitted light L3 passingthrough the penetration space 21, and reduce the light that possiblybecomes the noise light.

As illustrated in FIG. 7, a graining portion 23 may be formed in thesurface 18 a on the side of the printed board 15 in the regioncorresponding to the penetration space 21. The graining portion 23 canabsorb the emitted light L3 passing through the penetration space 21,and reduce the light that possibly becomes the noise light. The lightabsorption effect can further be enhanced by a combination of the use ofthe black color and the formation of the graining portion 23.

As illustrated in FIG. 8, the hole portion of the lower case 18 may be ahole portion 22 a constructed by a recess that does not penetrate thelower case 18 in the thickness direction. In this case, the generationof the noise light can further be reduced by the use of the black coloror the formation of the graining portion 23.

As illustrated in FIG. 9, the penetration spaces 21 are formed in theportions in which the light receiving elements 13 and 14 are attached tothe printed board 15 in addition to the portion in which the lightemitting element 12 is attached.

As illustrated in FIG. 9A, the hole-shaped penetration spaces 21 areformed in the portions in which the light emitting element 12 and thelight receiving elements 13 and 14 are attached. The penetration spaces21 have a rectangular shape when viewed from above, and penetrate theprinted board 15 in the thickness direction. The penetration spaces 21are formed while including the portions corresponding to chips 12 a, 13a, and 14 a of the light emitting element 12 and the light receivingelements 13 and 14, and the detail of the penetration space 21 isdescribed above.

In the case 16, the through-hole 22, which is the hole portionpenetrating the printed board 15 in the thickness direction, is alsomade in the lower case 18 with which the lower surface of the printedboard 15 is covered. As illustrated in FIGS. 9B and 9C, the through-hole22 is made only in the region corresponding to the penetration space 21that is formed below the light emitting element 12 of the printed board15. This is because the light is prevented from invading in the lightreceiving elements 13 and 14 from the outside of the lower case 18.

In the case where the hole portion is made in the region correspondingto the penetration space 21 below the light receiving elements 13 and14, the hole portion 22 a (see, FIG. 8) constructed by the recess thatdoes not penetrate the lower case 18 in the thickness direction asillustrated in FIG. 8. In this case, the generation of the noise lightcan further be reduced by the use of the black color or the formation ofthe graining portion 23 (see FIG. 7).

Even in the toner density sensor 11 having the above configuration, notonly the light emitted from the light emitting element 12 travels in thedirection of the lens member 19 as illustrated in FIG. 9B, but also thelight travels in the direction of the printed board 15 as illustrated inFIG. 9C.

As described above, the emitted light L1 travelling in the direction ofthe lens member 19 is transmitted through the lens member 19 andreflected by the transfer belt 56, and the reflected light L3 isreceived by the light receiving elements 13 and 14 through the lensmember 19, thereby detecting the toner density.

On the other hand, the emitted light L3 travelling from the lightemitting element 12 in the direction of the printed board 15 radiates tothe outside through the penetration space 21 of the printed board 15 andthe through-hole 22 of the lower case 18.

Although part of the emitted light invades in the printed board 15,because the emitted light L3 travelling in the direction of the printedboard 15 radiates substantially from the penetration space 21, theamount of noise light invading in the printed board 15 can be reduced.Even if the small amount of noise light invades in the printed board 15,the light attenuates in time. Since the penetration spaces 21 are alsoformed in the portions in which the light receiving elements 13 and 14are attached, the noise light diffuses and attenuates in the insidesurfaces of the penetration spaces 21. Because the penetration space 21is formed by pressing (punching) or drilling, the smooth cut surface isnot obtained, but the cut surface has an irregular surface. Therefore,the noise light reaching the light receiving elements 13 and 14 issignificantly reduced.

Accordingly, the light receiving elements 13 and 14 are hardlyinfluenced by the noise light, and the improvement of the detectionaccuracy can be achieved.

The penetration spaces 21 are formed in both the portion in which thelight emitting element 12 is attached and the portions in which thelight receiving elements 13 and 14 are attached. In the printed board15, because of the structure in which the penetration spaces 21 areformed below the light emitting element 12 and the light receivingelements 13 and 14, the necessity of the additional flat surface inwhich the penetration spaces 21 are provided is eliminated, and thesmall area can effectively be utilized. Therefore, thehigh-detection-accuracy, small-size toner density sensor 11 can beobtained.

In the lower case 18, because the through-hole 22 is not made below thelight receiving elements 13 and 14 while the through-hole 22 is madebelow the light emitting element 12, the large amount of light emittedfrom the light emitting element 12 radiates to the outside, and thenoise light reaching the light receiving elements 13 and 14 cansignificantly be reduced while the light that possibly becomes the noiselight is further reduced.

FIG. 10 illustrates another example of the lower case 18. The grainingportions 23 are formed in the surface 18 a on the side of the printedboard 15 in the regions corresponding to the penetration spaces 21formed below the light receiving elements 13 and 14. Therefore, thenoise light invading in the penetration spaces 21 below the lightreceiving elements 13 and 14 can be prevented from being reflectedagain. As a result, the light receiving elements 13 and 14 cansuccessfully be protected from the noise light.

As illustrated in FIG. 11, when a plated layer 24 is formed in theinside surface of the penetration space 21, the effect that prevents thegeneration of the noise light and the effect that prevents the noiselight from reaching the light receiving elements 13 and 14 can beenhanced.

That is, as illustrated in FIGS. 11A and 11C, the plated layers 24 areformed in the inside surfaces of the penetration spaces 21 formed belowthe light emitting element 12 and the light receiving elements 13 and14. The plated layer 24 can be formed by the same forming as that of thecase in which usually the through-hole is made.

Because the light is blocked by the plated layer 24 in the insidesurface of the penetration space 21, the noise light that invades in theprinted board 15 from the penetration space 21 can successfully bereduced in the penetration space 21 below the light emitting element 12.The transmission of the noise light that propagates in the printed board15 to go out to the penetration space 21 can be prevented in thepenetration spaces 21 below the light receiving elements 13 and 14.Therefore, the noise light reaching the light receiving elements 13 and14 is significantly reduced.

The plated layer 24 can also play the same role to prevent thegeneration of the noise light when the penetration space 21 is formedonly below the light emitting element 12 as illustrated in FIG. 4.

As illustrated in FIG. 12, the penetration spaces 21 are formed in theportions in which the light receiving elements 13 and 14 are attached.

As illustrated in FIG. 12A, the hole-shaped penetration spaces 21 areformed in the portions in which the light receiving elements 13 and 14are attached. The penetration spaces 21 have the rectangular shape whenviewed from above, and penetrate the printed board 15 in the thicknessdirection. The penetration spaces 21 are formed while including theportions corresponding to chips 13 a and 14 a of the light receivingelements 13 and 14, and the detail of the penetration space 21 isdescribed above.

The hole portion is not made in the case 16. As needed basis, thesurface on the side of the printed board 15 in the region correspondingto the penetration space 21 may be formed in the matte black color, orthe graining portion 23 may be made in the surface on the side of theprinted board 15 in the region corresponding to the penetration space21.

In the toner density sensor 11 having the above configuration, the lightemitted from the light emitting element 12 travels toward the directionof the lens member 19 as illustrated in FIG. 12B, and the light alsotravels in the direction of the printed board 15 as illustrated in FIG.12C.

As described above, the emitted light L1 travelling in the direction ofthe lens member 19 is transmitted through the lens member 19 andreflected by the transfer belt 56, and the reflected light L2 isreceived by the light receiving elements 13 and 14 through the lensmember 19, thereby detecting the toner density.

On the other hand, the emitted light L3 travelling from the lightemitting element 12 in the direction of the printed board 15 invades inthe printed board 15, and propagates onto the sides of the lightreceiving elements 13 and 14 while being reflected or attenuated by aboundary surface between the printed board 15 and the lower case 18.However, since the penetration spaces 21 are formed in the portions inwhich the light receiving elements 13 and 14 are attached, the noiselight diffuses and attenuates in the irregularity of the inside surfaceof the penetration space 21. Therefore, the noise light reaching thelight receiving elements 13 and 14 can be prevented.

Accordingly, the light receiving elements 13 and 14 are hardlyinfluenced by the noise light, and the improvement of the detectionaccuracy can be achieved.

The penetration spaces 21 are formed in the portions in which the lightreceiving elements 13 and 14 are attached. In the printed board 15,because of the structure in which the penetration spaces 21 are formedbelow the light receiving elements 13 and 14, the necessity of theadditional flat surface in which the penetration spaces 21 are providedis eliminated, and the small area can effectively be utilized.Therefore, the high-detection-accuracy, small-size toner density sensor11 can be obtained.

In the lower case 18, the hole portion is not made in the regionscorresponding to the penetration spaces 21 below the light receivingelements 13 and 14, so that the noise light can be prevented frominvading from the outside.

As described above, the noise light reaching the light receivingelements 13 and 14 is significantly reduced.

When the penetration spaces 21 are formed only in the portions in whichthe light receiving elements 13 and 14 are attached, the plated layer 24may be formed in the inside surface of the penetration space 21 asillustrated in FIG. 13. This is because the noise light can be preventedfrom invading in the penetration space 21 from the inside of the printedboard 15.

As illustrated in FIG. 14, some of the toner density sensors 11 do notinclude the lower case 18. In such cases, although the noise lightreducing effect is not obtained by the hole portion, the black color,and the graining portion 23 of the lower case 18, the adverse effect ofthe noise light can be reduced by the penetration space 21 that isformed in the portion in which at least one of the light emittingelement 12 and the light receiving elements 13 and 14 is attached.

In one or more embodiments of the present invention, the light emittingunit corresponds to the light emitting element 12, the light receivingunit corresponds to the light receiving element (the first lightreceiving element 13 and the second light receiving element 14), theboard corresponds to the printed board 15, the hole portion correspondsto the through-hole 22 and the hole portion 22 a. However, the inventionis not limited to the above embodiments, and another configuration maybe adopted.

In one or more embodiments, by way of example, the light emittingelement 12 and the light receiving elements 13 and 14 are disposed onthe same substantially straight line in order to achieve the small-sizetoner density sensor 11. Alternatively, for example, the light emittingelement 12 and the light receiving elements 13 and 14 may be disposedinto a V-shape in the toner density sensor. In this case, similarly theimprovement of the detection accuracy can be achieved.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

-   11 TONER DENSITY SENSOR-   12 LIGHT EMITTING ELEMENT-   13 FIRST LIGHT RECEIVING ELEMENT-   14 SECOND LIGHT RECEIVING ELEMENT-   12 a, 13 a, 14 a CHIP-   15 PRINTED BOARD-   16 CASE-   18 a SURFACE ON BOARD SIDE IN REGION CORRESPONDING TO PENETRATION    SPACE-   21 PENETRATION SPACE-   22 THROUGH-HOLE-   22 a HOLE PORTION-   23 GRAINING PORTION-   24 PLATED LAYER-   51 IMAGE FORMING APPARATUS

1. A toner density sensor comprising: a light emitting unit that emitslight to detect toner density; a light receiving unit that receives thelight emitted from the light emitting unit and reflected from adetection target; and a board on which the light emitting unit and thelight receiving unit are surface-mounted, wherein a penetration spacethat penetrates the board in a thickness direction is formed in at leastone of portions in which the light emitting unit and the light receivingunit are attached to the board.
 2. The toner density sensor according toclaim 1, wherein the penetration space is formed around a regioncorresponding to a chip portion of the light emitting unit or the lightreceiving unit.
 3. The toner density sensor according to claim 1,wherein a plated layer is formed in an inside surface of the penetrationspace.
 4. The toner density sensor according to claim 1, wherein a casewith which the light emitting unit and the light receiving unit arecovered is provided in the board, and a hole portion is made in a regioncorresponding to the penetration space in the case.
 5. The toner densitysensor according to claim 4, wherein the hole portion is a through-holethat penetrates the board in the thickness direction.
 6. The tonerdensity sensor according to claim 1, wherein a case with which the lightemitting unit and the light receiving unit are covered is provided inthe board, and a surface on the board side in a region corresponding tothe penetration space in the case has a matt black color.
 7. The tonerdensity sensor according to claim 1, wherein a case with which the lightemitting unit and the light receiving unit are covered is provided inthe board, and a graining portion is formed in a surface on the boardside in a region corresponding to the penetration space in the case. 8.An image forming apparatus on which the toner density sensor accordingto claim 1 is mounted.
 9. The toner density sensor according to claim 2,wherein a plated layer is formed in an inside surface of the penetrationspace.
 10. The toner density sensor according to claim 2, wherein a casewith which the light emitting unit and the light receiving unit arecovered is provided in the board, and a hole portion is made in a regioncorresponding to the penetration space in the case.
 11. The tonerdensity sensor according to claim 3, wherein a case with which the lightemitting unit and the light receiving unit are covered is provided inthe board, and a hole portion is made in a region corresponding to thepenetration space in the case.
 12. The toner density sensor according toclaim 9, wherein a case with which the light emitting unit and the lightreceiving unit are covered is provided in the board, and a hole portionis made in a region corresponding to the penetration space in the case.13. The toner density sensor according to claim 10, wherein the holeportion is a through-hole that penetrates the board in the thicknessdirection.
 14. The toner density sensor according to claim 11, whereinthe hole portion is a through-hole that penetrates the board in thethickness direction.
 15. The toner density sensor according to claim 12,wherein the hole portion is a through-hole that penetrates the board inthe thickness direction.
 16. The toner density sensor according to claim2, wherein a case with which the light emitting unit and the lightreceiving unit are covered is provided in the board, and a surface onthe board side in a region corresponding to the penetration space in thecase has a matt black color.
 17. The toner density sensor according toclaim 3, wherein a case with which the light emitting unit and the lightreceiving unit are covered is provided in the board, and a surface onthe board side in a region corresponding to the penetration space in thecase has a matt black color.
 18. The toner density sensor according toclaim 9, wherein a case with which the light emitting unit and the lightreceiving unit are covered is provided in the board, and a surface onthe board side in a region corresponding to the penetration space in thecase has a matt black color.
 19. The toner density sensor according toclaim 2, wherein a case with which the light emitting unit and the lightreceiving unit are covered is provided in the board, and a grainingportion is formed in a surface on the board side in a regioncorresponding to the penetration space in the case.
 20. The tonerdensity sensor according to claim 3, wherein a case with which the lightemitting unit and the light receiving unit are covered is provided inthe board, and a graining portion is formed in a surface on the boardside in a region corresponding to the penetration space in the case.